The present invention relates to an image-pickup apparatus such as a digital still camera or a digital video camera having an electronic distortion correction function and relates to an interchangeable lens mounted to the image-pickup apparatus.
The image-pickup apparatus capturing an image (image data) using a solid-state image-pickup element such as a CCD sensor or a CMOS sensor includes an image-pickup optical system equipped thereto or detachably mounted thereto. Along with a trend for the miniaturization of the image-pickup apparatus, the image-pickup optical system also is being made miniaturized. The miniaturization of the image-pickup optical system, however, makes it more difficult to correct optical aberrations sufficiently. Thus, the captured image data often has to be subjected to the processing of electronically correcting the aberration component thereof.
Japanese Patent Laid-Open No. 2005-286482 discloses a method of, based on an image-height vs. distortion data indicating a relationship between a coordinate position of image data and a distortion at each image height, correcting a distortion component of the image data due to the distortion in the image-pickup optical system.
Japanese Patent Laid-Open No. 2006-270918 discloses a method of correcting a distortion component for image data using data indicating a distortion corresponding to a zoom position and an object distance of an image-pickup optical system.
The distortion correction methods disclosed in Japanese Patent Laid-Open No. 2005-286482 and Japanese Patent Laid-Open No. 2006-270918, however, try to achieve accurate distortion correction over the entire zoom range of the image-pickup optical system, which causes deterioration in image quality instead.
Note here that a distortion amount at each image height of an image-pickup optical system, in general, changes with a zoom position of the image-pickup optical system. For instance, a zoom position closer to the wide-angle end means an increase in the distortion amount.
On the other hand, the distortion correction processing of image data is basically a coordinate transformation processing in which each coordinate (each pixel) of image data including a distortion component is transformed into a coordinate from which the distortion component has been removed or decreased, the distortion component being obtained by the calculation based on data concerning the distortion. Such coordinate transformation processing leads to a decrease in resolution of the corrected image data, i.e., deterioration in image quality.
In addition, the accurate distortion correction performed over the entire zoom range of the image-pickup optical system results in a large load on a calculating circuit that performs calculation for the correction or an increase in memory capacity that stores data relating to the distortion used for the calculation.
Especially in the case of an image-pickup apparatus required to output a real-time image with continuity rather than a high-quality image, such as in the case of a monitoring camera, excessive distortion correction processing and a large calculation load may degrade the performance as the image-pickup apparatus.